The aim of this study was to examine the reliability of peak force (PF) and rate of force development (RFD) from the Isometric Mid-Thigh Pull (IMTP) in 59 male basketball players from a Japanese professional club's youth academy classified into three age categories (U12: n = 18, U15: n = 27, and U18: n = 14). Each athlete completed three IMTP sessions, from which PF and mean RFD were calculated over time intervals of 0-50, 0-100, 0-150, 0-200, and 0-250 ms. Within-session and between-day reliabilities were evaluated using the intraclass correlation coefficient (ICC), coefficient of variation (CV), and standard error of measurement (SEM). PF demonstrated consistently high reliability across all age categories for both within-session and between-day analyses (ICC 0.77-0.98, CV 3.4-8.2%), with small SEM values (7.65-91.86 N, 0.60-4.13%) indicating limited measurement error. In contrast, RFD displayed poor reliability in shorter time windows (0-50 and 0-100 ms; ICC < 0.50), particularly in categories U12 and U15. Although longer time windows (0-150 and 0-250 ms) achieved higher reliability (ICC > 0.75) and lower SEM values than those previously reported, the CV values remained high (10.7-34.8%). Such a large variability may obscure true performance changes. Consequently, the PF is recommended as a robust metric for young athletes across all age groups. In contrast, the current protocol limits the ability of RFD to accurately detect individual changes. Therefore, RFD requires careful interpretation and protocol modifications, such as extended familiarization, to improve stability and precision.